The present invention relates to an electrostatographic apparatus such as a facsimile receiver.
An apparatus of this type comprises a transfer medium such as a plastic dielectric sheet about 100 microns thick with a thin layer of a conductive material formed on a back surface thereof. In response to, for example, electrical signals from a facsimile transmitter, a discharge electrode assembly comprising a plurality of discharge electrodes is energized to form an electrostatic image on the sheet through electrical discharge. The sheet is moved adjacent to the electrode assembly. The electrodes, which are arranged in a row, are individually energized in accordance with the input signals to progressively form the electrostatic image on the sheet. The sheet is reusable and is advantageously provided in the form of a rotating endless belt.
A developing unit comprising, for example, a magnetic brush applies a toner substance to the belt to develop the electrostatic image into a toner image. A transfer unit transfers the toner image to a copy sheet to which the toner image is fixed to provide a hard facsimile copy or reproduction.
In facsimile transmission, it is desirable to selectively transmit at a plurality of different speeds or data rates. The operations of forming the electrostatic image and transferring the same to a copy sheet must be carried out at corresponding speeds. For example, a high transmission rate provides low transmission time but low resolution, and vice-versa. Different original documents for facsimile transmission have different resolution requirements, as is well known in the art.
With all other factors being equal, increasing the speed of the belt to accommodate a higher transmission rate results in a reduction in the density of the copy. In the prior art, the voltage applied to the discharge electrodes is increased to restore the copy density to the desired level. However, there is a practical limit to how much the voltage may be increased, taking into effect safety of operation and damage to the electrodes. The voltage range required in the prior art is generally greater than can be accommodated in actual practice, and a choice must be made as to whether insufficient copy density is more objectionable than short electrode life at high transmission rates.